The role of ellipticity on the preferential binding site of Ce and La in C78-D3h—A density functional theory study

Abstract

Endohedral metallofullerenes that encapsulate one or several atoms, or a cluster of atoms have molecular properties making them useful both in technology and in bio-medical applications. Some fullerenes are found to have two metal atoms incarcerated and it has been recently found that two Ce atoms are incorporated into the C₇₈-D_3h (78 : 5) cage. In this study, we report calculations on the structural and electronic properties of Ce₂@C₇₈ using density functional theory (DFT). While Ce₂@C₈₀-I_h (D_3d) and La₂@C₈₀-I_h (D_2h) have different ground state structures, we have found that Ce₂@C₇₈ has a D_3h ground state structure just as La₂@C₇₈. The encapsulated Ce atoms bind strongly to the C₇₈-D_3h cage with a binding energy (BE) of 5.925 eV but not as strong as in Ce@C₈₂-C_2v nor in Ce₂@C₈₀-I_h. The elliptical nature of the cage plays a crucial role and accommodates the two Ce atoms at opposite ends of the C₃ axis with a maximized inter atomic distance (4.078 Å). This means that the effect of the additional f-electron repulsion in M₂@C₇₈ with M = Ce compared to M = La, is less pronounced than in Ce₂@C₈₀ compared to La₂@C₈₀. We compare the results to the elliptical M₂@C₇₂ (#10611) (M = La, Ce), and with a range of additional Ce and La endohedral fullerenes and explain the role ellipticity has in the preferential binding site of Ce and shed light on the formation mechanism of these nanostructures.